1. Field of the Invention
The present invention relates to a step-down DC-to-DC converter, and more particularly to a non-insulated step-down DC-to-DC converter including a step-down switching regulator.
2. Discussion of the Background
A non-insulated step-down DC-to-DC (direct current to direct current) converter that uses an inductor such as a step-down-type switching regulator is grouped into two different types: a synchronous type and an asynchronous type. In general, such a step-down DC-to-DC converter has a continuous mode in which the inductor continuously produces a current to flow therethrough at an application of a relatively heavy load that produces a relatively large load current. The step-down DC-to-DC converter further has an intermittent mode in which the inductor intermittently produces the current to flow therethrough at an application of a relatively small load that produces a relatively small load current.
FIG. 1 illustrates an example output circuit of an example background synchronous-type non-insulated step-down DC-to-DC converter (hereinafter referred to as a synchronous step-down DC-to-DC converter). A synchronous step-down DC-to-DC converter having such an output circuit, as shown in FIG. 1, generally achieves a relatively high efficiency in the continuous mode. However, it decreases the efficiency to an extreme extent in the intermittent mode. This is because, in the intermittent mode, the output circuit produces a reverse current that flows from the load side to the ground through a transistor M102 for synchronous rectifying.
Thus, when the load to the step-down DC-to-DC converter is reduced to a relatively light level, a switching transistor M101 and the transistor M102 for synchronous rectifying frequently perform switching. Accordingly, switching losses of the switching transistor M101 and the transistor M102 increase. To reduce the switching losses, the above step-down DC-to-DC converter changes a transistor control from a PWN (pulse-width modulation) control to a PFM (pulse frequency modulation) control.
Furthermore, when the step-down DC-to-DC converter changes its mode to the PFM control, it changes such that the output circuit forms an asynchronous rectifying so as to prevent a reduction of efficiency due to a reverse current. As illustrated in FIG. 2, a diode D101 is generally used as a rectifying element in an output circuit of an asynchronous-rectifying-type step-down Dc-to-DC converter. In the circuitry of FIG. 2, a voltage provided to the load reversely biases the diode D101 even in the intermittent mode, thereby preventing the reverse current. However, the diode D101 has a relatively large electric power consumption and is not capable of increasing efficiency because it has a relatively large forward voltage in the order of approximately 0.6 volts.
FIG. 3 illustrates another example asynchronous-type step-down DC-to-DC converter which has been improved in efficiency. In FIG. 3, a bipolar transistor Q101 for switching, i.e., a PNP transistor, has a base to which a PWM comparator (not shown) sends an output signal, a drive signal. When the drive signal is turned to a high level and the bipolar transistor Q101 is subsequently turned off, a voltage V101 appearing at one end of a inductor L101 is lowered down to a negative voltage. In a comparator CMP101, a non-inverse input terminal is connected to a point of a ground voltage and an inverse terminal is connected to a point of the voltage V101 where the bipolar transistor Q101 and the inductor L101 are connected to each other. Thus, the comparator CMP101 exhibits a hysteresis.
When the voltage V101 is lowered to a low level, the comparator CMP101 outputs a high level signal from an output terminal. This causes a MOS transistor M102 used as a rectifier to be turned on since the MOS transistor M102 has a gate connected to the output terminal of the comparator CMP101. Thereby, the inductor L101 generates no current, i.e., 0 amperes and the voltage V101 is increased. When the voltage V101 is raised to the ground voltage or above, the comparator CMP101 outputs a low level signal from the output terminal. This causes the MOS transistor M102 to be turned off so as to prevent an input of the reverse current from the load. To increase an efficiency during an asynchronous rectifying, it is effective to use a MOS transistor having a resistance in an on-state smaller than that of the diode D101 (i.e., a Schottky diode) in place of the MOS transistor M102.
As shown in FIG. 3, this circuit uses the comparator CMP101 to control the rectifying MOS transistor M102. In this case, the circuit causes a delay in operations until the MOS transistor M102 is turned on after the voltage V101 is reduced down to the negative voltage. To suppress such a delay in operations, the Schottky diode D101 is provided and, therefore, the rectifying MOS transistor M102 and the comparator CMP101 are added to the circuit of FIG. 2. That is, the circuit of FIG. 3 involves another drawback of an increase in an area of the circuit.